This invention relates to superheterodyne radio receivers using frequency synthesizing apparatus to develop the required local oscillator signal and, in particular, to such receivers capable of operation on multiple frequency bands wherein the frequencies in one band are spaced from each other by frequency increments which are different from the frequency increments by which frequencies on another band are spaced from each other and to such receivers wherein the synthesizing apparatus reference frequency is greater than the frequency increment between adjacent local oscillator frequencies required by the receiver. The invention further finds particular applicability in the provision of radio receivers for receiving signals on all frequencies designated by the United States Federal Communications Commission as being available for assignment to the Public Safety Radio Services.
Radio receivers using frequency synthesizer apparatus to generate the required local oscillator signals are well known. Such receivers of modern design conventionally use phase-locked loop circuits as part of the frequency synthesizing apparatus. The phase-locked loop circuit provides the advantages of extreme ease of tuning, stability of operation, and capability of generating signals at a large number of frequencies. Such receivers are discussed, for example, in "RCA Solid State Databook Series: COS/MOS Digital Integrated Circuits," No. SSD-203A, published by RCA Corporation, Box 3200, Somerville, New Jersey, particularly at pages 419-433. Further, of course, receivers useful for reception of signals on a plurality of different frequency bands are also well known. Certain difficulties have arisen, however, in the design and construction of multiband radio receivers using frequency synthesis techniques; those difficulties are particularly troublesome in cases where it is desired to construct such a radio for receiving signals on bands which are widely separated in frequency and on bands in which the signal in the different bands are separated by different frequency increments.
One example where such difficulties have arisen is in the design of receivers for reception of signals on the radio frequencies designated by the United States Federal Communications Commission as being assigned to the Public Safety Radio Service. Radio receivers are known which are capable of automatically and sequentially tuning to a plurality of such frequencies. Those receivers have most often included a crystal controlled oscillator with individual crystals for generating the local oscillator signals required for reception of signals at each of the tuned frequencies. Examples of such receivers are the multichannel scanning receiver disclosed in the copending application of G. H. Fathauer filed on Nov. 4, 1974, Ser. No. 520,438, now U.S. Pat. No. 3,987,400 and the receivers discussed in the patents and application cited therein. Receivers such as those have had the advantages of extreme ease of tuning and stability of operation; however, they have had the disadvantage that they required one crystal for each frequency to be received. The cost of crystals can become a significant amount where a receiver is to be capable of receiving signals at a large number of different frequencies. Moreover, when it is desired to alter one of the received frequencies, the crystal must be changed, resulting in further expense and inconvenience to the user. Receivers using frequency synthesizers to generate the local oscillator signals provide ease of tuning and stability of operation similar to that of crystal receivers, but have the advantage that separate crystals are not required for each frequency to be received. Moreover, when a frequency is to be changed, this may be accomplished by relatively simple changes to the phase-locked loop circuitry. Receivers can be designed so that these changes may be easily made by a relatively unskilled user.
However, as will be subsequently explained in greater detail, the public service radio frequencies fall in relatively widely spaced frequency bands, the lowest being in the band of 30-50 MHz. and the highest in the band of 470-512 MHz. Moreover, the frequencies in the different bands occur at frequency increments of 15 kHz., 20 kHz., 25 kHz., and 50 kHz. in the different bands. If previously known frequency synthesizing techniques were used to generate the local oscillator signals required for reception of signals on all the public radio service frequencies, circuits of undesirable complexity and expense would result.